They don't really HAVE to think about the various instances imo. They just need to join one, that's it. Following users/communities from other instances isn't hard to wrap your head around, you just follow them. badda-bing badda-boom. The @instance.whatever bit of their username barely matters. You just say "that's like a URL to find that user on a different instance than the one you chose". People arent as stupid as you might think, they just need someone patient enough to explain.